This invention was made with government support under Contract No. DE-AC06-76RLO 1830 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
Electrolytic cells, such as a Hall-Heroult cell for aluminum production by the electrolysis of alumina in molten cryolite, conventionally employ conductive carbon electrodes. The Hall-Heroult process reduces aluminum metal from alumina in a molten salt electrolyte and consumes carbon from a carbon anode in the process. The anode liberates oxygen from the alumina, which results in aluminum metal being collected on the cathode. The oxygen combines with carbon to produce CO and CO.sub.2. The overall reaction in its simplest form is represented as follows: ##STR1## Approximately 0.33 pounds of carbon are consumed for every pound of aluminum produced. The life of carbon anodes is typically two to three weeks.
Carbon obtained from petroleum coke is typically used for fabrication of such anodes. Such material is becoming increasingly expensive. The petroleum coke also typically contains significant quantities of impurities such as sulfur, silicon, vanadium, titanium, iron and nicke. Such impurities can contaminate the metal being produced as well as cause environmental problems and poor working conditions. Removal of excess quantities of such impurities requires extra and costly steps when high purity aluminum is to be produced.
If no carbon were consumed in the reduction of alumina, the overall reaction would be: 2Al.sub.2 O.sub.3 .fwdarw.4Al +30.sub.2. Accordingly, non-consumable anodes could be used in a process where carbon does not enter into the electrolytic reaction. Such anodes would have a life limited only by corrosion due to the caustic cryolite electrolyte and electrochemical degradation mechanisms. It is anticipated that the life of such anodes could be extended to several months or even a year or more as compared to the two to three week life of a carbon anode which is consumed in the electrolytic reduction reaction. Furthermore, non-consumable anodes would presumably not add the same significant quantities of impurities as do carbon anodes.
Numerous attempts have been made to develop an inert electrode, but apparently without the required degree of success to make it commercially feasible. The entire aluminum industry still uses consumable carbon anodes in the production of aluminum. Many of the newly developed inert electrodes apparently are reactive or corroded by the electrolyte to an extent which results in contamination of the metal being produced as well as consumption of the anode due to corrosion.
There have been numerous suggestions for non-consumable anode compositions based on various ceramic oxides and oxy compounds. Such materials typically behave as a semi-conductor having inherently low electrical conductivities on the order of 1 ohm .sup.-1 cm.sup.-1. (All electrical conductivities referred to in this document related to conductivity at normal cell operating temperatures of 900.degree. C. to 1,000.degree. C.) Attempts have been made to fabricate non-consumable electrodes with special compositions known as cermets. A cermet composition includes both metallic and ceramic phases. Cermets typically have higher electrical conductivity than pure ceramic compositions, and improved corrosion resistance as compared to metals. The conventional method of preparing cermet compositions is to mix metal and ceramic powders. cold press a preform, and sinter the preform at an elevated temperature in a controlled atmosphere. Alternatively, the cermet can be prepared by hot pressing or hot isostatic pressing wherein the sintering operation is carried out under pressure. Other densification methods for forming oxides and metals into cermets may also be usable.
One promising oxide system identified for use with cermets is the NiO-NiFe.sub.2 O.sub.4 system. However, most cermets using this oxide system, or other oxide systems, still have low electrical conductivities, on the order of 1 to 10 ohm.sup.-1 cm.sup.-1. Prior art oxide systems containing nickel in the metal phase (a NiO-NiFe.sub.2 O.sub.4 -Ni cermet) have been fabricated by reaction sintering processes and exhibit excellent electrical conductivity on the order of 300 ohm.sup.-1 cm.sup.-1. This is even an improvement over the conductivity of carbon anodes which typically average approximtely 200 ohm.sup.-1 cm.sup.-1. However, typical cermets have a discontinuous metal phase distributed throughout the oxide phase and are susceptible to anodic dissolution by corrosion. This causes anodes of such material to fail prematurely and also contaminate the produced aluminum with nickel.
Copper has also been incorporated into the NiO-NiFe.sub.2 O.sub.4 matrix creating an NiO-NiFe.sub.2 O.sub.4 -Cu cermet. The Cu metal phase is discontinuously distributed within the oxide matrix, but still provides improved electrical conductivity on the order of 60 to 70 ohm.sup.-1 cm.sup.-1. Such a material had a copper content of 17 weight percent.
For example, U.S. Pat. No. 4,620,905 to Tarcy et al. discloses an NiO-NiFe.sub.4 O.sub.4 -Cu-Ni cermet wherein 17% of the composition is comprised of a metal alloy of copper and nickel. The nickel metal is understood to arise primarily from the reduction of excess NiO in the oxide phase induced by the presence of carbon-based binders used to produce the oxide powders (col. 5, lines 3-14).
U.S. Pat. Nos. 4,374,761; 4,478,693; 4,399,008; and 4,374,050 to Ray and 4,455,211 to Ray et al. also disclose non-consumable cermet electrodes for use in molten salt electrolysis. The electrodes disclosed are stated to be comprised of ceramic oxide compositions having at least one metal powder disbursed therethrough for purposes of increasing electrical conductivity. The metal powder is stated to be selected from the group consisting of Co, Fe, Ni, Cu, Pt, Rh, In, and Ir or alloys thereof. The metal is also stated to be provided in the electrode composition in amounts not constituting more than 30 volume percent metal. Additionally, elemental copper is stated to be includable in an amount up to 30 weight percent of the finished composition using the Ray processes. However, no example in any of these patents supports the broad statements concerning achieving high metal content in a cermet. Further, the metal is indicated as being coated with a wax binder to prevent the metal particles from oxidizing during the sintering step. Additionally, the electrical conductivities of the example electrodes range from 0.4 ohm.sup.-1 cm.sup.-1 to 32 ohm.sup.-1 cm.sup.-1.
Prior to the present invention, 17% metal alloy of copper and nickel was understood to be at or close to the practical upper limit of including an alloy of copper and nickel or pure copper within a NiO-NiFe.sub.2 O.sub.4 oxide system, despite statements in the above prior art patents. Apparently in these processes, additions of copper above this limit merely bled out of the composite during sintering, forming small copper metal beads on the surface of the cermet. Further, the alloy phase is discontinuously distributed throughout the oxide phase.